Fluorescent lamps utilizing a dual phosphor layer of different phosphors to produce a high lumen output together with good color rendition are well known. This invention deals with a particular dual phosphor layer improvement in fluorescent lamps to provide these desired results more effectively than found in conventional lamps. More particularly, the present dual phosphor fluorescent lamps provides still higher luminous efficacy without appreciably sacrificing CRI (Color Rendering Index) values.
Efficient illumination along with good color rendition is now provided in fluorescent lamps with various combinations of three narrow band emitting phosphor materials each selectively producing red, blue or green color emission. For example, there is disclosed in U.S. Pat. No. 3,937,998 a combination of said type phosphor materials found useful in low pressure type mercury vapor discharge lamps wherein the blue color emission phosphor exhibits an emission band in the wavelength range from about 430 nanometers wavelength up to about 490 nanometers wavelength, the red color emission phosphor exhibits an emission band in a wavelength range from about 590 nanometers wavelength up to about 630 nanometers wavelength, and with the green color emission phosphor producing emission extending from about 520 nanometers wavelength up to about 565 nanometers wavelength, said phosphor combination producing efficient white color illumination. In U.S. Pat. No. 4,088,923 there is also disclosed a particular utilization of said phosphor combination as a top layer in a dual layer phosphor coating which is said to reduce the cost of these relatively expensive phosphor materials in a fluorescent lamp. Specifically, a thin layer of this tri-phosphor combination can be deposited on the surface of a conventional calcium haloapatite phosphor material to produce the desired high light output with good color rendition at a considerable cost savings for the overall phosphor combination.
Various other narrow band emitting phosphor materials which are relatively less expensive than the above referenced high cost phosphors can also be employed in this type phosphor combination. For example, a green color emitting terbium-activated lanthanum cerium orthophosphate phosphor is disclosed in U.S. Pat. No. 4,423,349 as a lower cost substitute for the terbium and cerium coactivated magnesium aluminate phosphor component in said phosphor combination. Similarly, a lower cost europium-activated strontium chloroapatite phosphor can be substituted for europium-activated barium magnesium aluminate phosphor to serve as the narrow band blue color emission phosphor component in a different phosphor combination used to provide efficient white color illumination in a fluorescent lamp. Specifically, the latter type phosphor coating is disclosed in U.S. Pat. No. 4,075,532 wherein the improved coating comprises a first phosphor having a relatively broad emission spectrum with a mean wavelength in the yellow portion of the visible spectrum which is blended with a second phosphor having said relatively narrow emission spectrum in a blue portion of the visible spectrum and with said first and second phosphors being uniformly blended together in a proportional relationship preselected to provide increased luminous efficacy for said mixture. Although the color rendering capability for the latter phosphor mixture has been found commercially acceptable, it is recognized to be significantly lower than that achieved with the conventional calcium haloapatite phosphor material.
It is a principal object of the present invention, therefore, to provide still further improved white color illumination in fluorescent lamps than achieved in the conventional lamps.
It is another important object of the present invention to provide higher output white color illumination in fluorescent lamps without appreciably sacrificing CRI values.
Other objects of the present invention will become apparent to those skilled in the art upon considering the detailed description hereinafter provided.